An essential goal of the nervous system is to maintain orientation and equilibrium relative to the outside world. Attaining this goal requires that we accurately localize ourselves within our surroundings, distinguish self-movement from that of environmental elements, and control movement through a complex world. These tasks are accomplished by neural systems that coordinate multiple motor behaviors with the aid of feedback from several sensory modalities. Vision and audition (the exteroceptive senses) provide information about the position and motion of external elements, proprioception conveys the position of the body and its parts, and vestibular inputs track head motion and orientation. These sensory inputs are processed by the brain, which generates the motor behaviors required to perform natural activities. Examples include coordinated eye and head movements that acquire and hold fixation on targets of interest, and guided hand movements that allow us to accurately interact with objects in the environment. This project will elucidate how these crucial and ubiquitous functions work and maintain coordination, and how they degenerate with natural aging. The endeavor is motivated by two important and related public health concerns: aging is accompanied by a progressive increase in the prevalence of disequilibrium, and in the risk and consequences of falls. Disequilibrium and falls both constitute failures of spatial orientation and equilibrium control. We hypothesize that the key to understanding these failures lies in how the aging nervous system utilizes multiple sensory inputs, central integrative processes, and adaptive mechanisms to produce and maintain cross- sensory concordance in the brain's representation of space. We will study physiological and psychophysical parameters that underlie how the brain utilizes vestibular, visual, and auditory inputs to localize external objects, and to control eye, head, and hand movements in order to acquire them. We will also elucidate how these sensory modalities interact to register motion of the body and external elements. Further, we will quantify adaptive capabilities that maintain accurate calibration between sensory modalities and their respective depictions of space. State-of-the-art and innovative techniques will be employed to isolate and combine specific sensory cues relevant to spatial localization and orientation, and over a broad range of ages. We hypothesize that aging entails quantifiable deterioration in spatial tasks, especially those that require integrative processes across sensory modalities. Further, we hypothesize that aging will result in quantifiable limitations in adaptive capabilities that are required to restore normal function in the presence of disease or age-dependent deterioration. A thorough understanding of these functions is required to facilitate the development of specific and effective interventions that might correct, or rehabilitate, potential age or disease induced deficits.